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the Williams Fork River, the average dissolved solids concentration is <br />311 mg/1. Dissolved solids concentration of the Yampa River average <br />264 mg/1 upstream from the confluence with the Williams Fork River and <br />284 mg/1 downstream. <br />Water within the Yampa River Basin is consumed through the irrigation of <br />croplands, municipal water supplies, stock watering, a~oling water for <br />power plants, evapotranspiration by riparian vegetation and phreatophytes, <br />and transbasin diversions. Irrigation of cropland con[:titutes the largest <br />of these uses. Surface water consumed within the Yampa River drainage <br />basin in 1976 totaled approximately 445,000 acre-feet. Of that, 399,000 <br />acre-feet were used for irrigating croplands and hay meadows or for <br />watering livestock. Other uses included 5,478 acre-feet for industrial <br />purposes, 2,555 acre-feet for municipal water supplies, and 8,283 acre-feet <br />for other unspecified uses (Steele et al., 1979). Industrial consumption <br />has since increased by a total of 18,720 acre-feet per year due to use by <br />the Craig generating station. <br />Water for agricultural irrigation ie generally obtained. by simple stream <br />diversion structures and networks of ditches for flooding grasslands and <br />meadows during summer months. The short growing season precludes growing <br />of warm weather crops such as corn. In the Williams Fork River, an even <br />higher proportion of the water used ie for irrigation of grasslands and hay <br />fields. <br />Impacts of Mine Dewatering on Stream Depletion <br />Because the coal has a relatively small subcrop length under the alluvial <br />valleys and because the alluvium has a very low permeability, very little <br />interchange of water between the coal and river ie expe~=ted. An attempt <br />has been made to prepare a worst-case estimate of the f:Low from the river <br />to the mines. The estimates were made using an equatirn~ developed by <br />McWhorter (1981). <br />The total worst-case estimated stream depletion is 182 qpm. During mining, <br />the discharge from the mines will be many times this ameunt; therefore, <br />during mining, the net effect will be an increase in stream flow. <br />The mine dewatering ie presently not causing any measureible depletion of <br />stream flows in the mine vicinity. Even if all of the present mine inflows <br />(approximately 2 cfs) were coming from the rivers, it would be too small to <br />measure even at 7-day/SO-year low flows. <br />&ffects of Mine Discharge on Surface Water Quality <br />To characterize the effects of mine discharge on the Williams Fork River, a <br />mass balance analysis for mean monthly dissolved solids and SAR was done <br />and can be found in Exhibit 34 of the permit application package. The <br />calculated effect of the mine discharge and Strip Pit discharge is to <br />increase dissolved solids concentrations during a low flow event from 500 <br />mg/1 to 570 mg/1 and to increase SAR from 0.5 to 2.8. The Williams Fork <br />River water would still maintain a low sodium hazard and medium salinity <br />hazard during low flows. Therefore, the mine discharge should not signifi- <br />cantly impact the agricultural usefulness of the water. <br />Water quality data have been collected upstream and downstream of the mine <br />site on the Williams Fork River on a quarterly basis sin=e 1981. Monthly <br />data are available for pH, conductivity, and temperature. <br />26 <br />